394 Evolution and Adaptation 



night, because they are positively heliotropic to weak light, 

 and also because they are negatively geotropic. They go 

 down during the day, because they react to bright light 

 more strongly than to gravity. The males follow the fe- 

 males, largely because they react positively chemotactically 

 toward the females. 



Some other animals respond in a somewhat different way 

 to light, as shown by the fresh-water planarians. These 

 animals remain during the day under stones, where the 

 amount of light is relatively less than outside. If they are 

 placed in a dish in the light in front of a window, they crawl 

 away from the light, but when they reach the back of the 

 dish they do not come to rest, but continue to crawl around 

 the sides of the dish even toward the light. The light 

 makes the worms restless, and while they show a negative 

 response as long as they are perfectly free to move away 

 from the light, they will not come to rest when they come 

 to the back of the dish if they are there still in the light, 

 because the irritating action of the light on them is stronger 

 than its directive action. If, however, in crawling about 

 they come accidentally into a place less bright than that in 

 which they have been, they stop, and will not leave this 

 somewhat darker spot for a brighter one, although they 

 might leave the newly found spot for one still less bright. 



At night the planarians come out and wander around, 

 which increases their chance of finding food, although it 

 would not be strictly correct to say that they come out in 

 search of food. If, however, food is placed near them, a 

 piece of a worm, for example, they will turn toward it, 

 being directed apparently by a sense of smell, or rather of 

 taste. 



The heliotropic responses of the planarians appear to be of 

 use to them, causing them to hide away in the daytime, and 

 to come out only after dark, when their motions will not dis- 

 cover them to possible enemies. But some of the planarians 



